Semiconductor optical modulators can provide high speed modulation in combination with low voltage and power requirements. Such modulators are of interest for various applications, such as telecommunications and optical interconnects for large scale electronic integrated circuits.
Accordingly, optical modulators have been extensively investigated in the art. Semiconductor modulators often include quantum wells in the device active region to improve modulation performance. It is customary to classify such modulators according to the geometrical relation between the light being modulated and the quantum wells. One possibility is to have light propagating essentially perpendicular to the quantum wells, e.g., as considered in U.S. Pat. No. 4,525,687. Such surface normal devices have short interaction lengths, which undesirably increases modulator switching voltage. In many cases, the resulting modulator switching voltages are sufficiently high that the device is not compatible with mainstream electronics (e.g., silicon CMOS). Surface normal devices disposed in a vertical optical resonator to improve modulator performance have been considered, e.g., in U.S. Pat. No. 5,488,504.
Another possibility is to have light propagating in the plane of the quantum wells. Long interaction lengths can be obtained in this geometry, typically by including the quantum well active region in an single mode optical waveguide, e.g., as considered in U.S. Pat. No. 5,522,005. However, coupling into such single mode waveguides raises considerable difficulties in practice. In particular, the small mode size and differing horizontal and vertical divergence angles typical of semiconductor waveguides create significant complications for input and output coupling (e.g., to optical fibers).
It is also possible for the light to propagate through the modulator active region at a shallow angle with respect to the quantum wells. A quasi-waveguide angled-facet electroabsorption modulator (QWAFEM) has been considered by Helman et al. in an article “Misalignment-Tolerant Surface Normal Low-Voltage Modulator for Optical Interconnects” (IEEE JSTQE, 11(2), pp 338-342, March/April 2005). In a QWAFEM, light is coupled into and out of the device via the top surface. Within the device, internal reflections from angled faces define an optical beam path that passes through the quantum well region at a shallow angle. The active region is parallel to a bottom surface of the device, which also acts as an internal reflector defining the optical beam path. Thus a QWAFEM provides a longer interaction length than conventional surface normal devices, without coupling to a single mode semiconductor waveguide.
Although the surface normal input and output coupling characteristic of QWAFEMs can be advantageous in certain cases (e.g., for free space optical interconnects), it can also be disadvantageous. For example, it is often desirable to integrate optical modulators with conventional electronic circuitry. Typically, the top surface of a conventional integrated circuit is employed for making electrical connections and the bottom surface is employed for removing heat, or vice versa. Thus performing optical coupling via the top or bottom surfaces of a modulator can undesirably interfere with electrical connections and/or heat removal.
Accordingly, it would be an advance in the art to provide an optical modulator having increased interaction length without waveguiding and without requiring surface normal coupling.